234 bulletin: museum of comparative zoology. 



young cranidia in the pusillus stage. They have four pairs of glabellar 

 furrows, long eyes, and a wide brim. The adult has four pairs of 

 furrows, short eyes well forward, and no brim. Hicks, in the descrip- 

 tion, says of these specimens : — "In the young the margin is equal all 

 around, and a considerable space, also, separates the glabella from the 

 anterior margin. This space gradually diminishes as the individual 

 grows; and the glabella enlarges until, as in the fully grown species, 

 the margin becomes fully obliterated." 



America. 



Paradoxides tenellus Billings from Newfoundland is another form 

 described from a young specimen in the pusillus stage. The typical 

 cranidium is 6 mm. long, shows four pairs of furrows on the glabella, 

 has long palpebral lobes and a wide brim. This species may possibly 

 be the young of P. decorus, a very imperfectly known species which 

 Billings describes from a cranidium about 26 mm. long, and which 

 has four pairs of furrows, but has the glabella in contact with the rim. 

 It occurs in the same locality as P. tenellus. 



Matthew presents notes on the young of P. cteminicus and P. 

 acadius, and figures three specimens of these species in the pusillus 

 stage. All have wide brims, long palpebral lobes, and four pairs of 

 furrows. They are the young of forms whose glabellas nearly or quite 

 touch the anterior rim, and which retain the four pairs of furrows and 

 long palpebral lobes at maturity. 



Summary on Ontogeny. 



From the above survey of the material now available for the study 

 of the ontogeny of Paradoxides, we see that the youngest shell or 

 protaspis is very similar to that of Olenellus. The glabella in the 

 youngest specimens of both species of "Hydrocephalus" is specialized 

 and unlike that of any other trilobite of which the young is known, in 

 that it occupies a large part of the head, is very wide, and bears no 

 transverse furrows. The first furrow to appear is a median longi- 

 tudinal one, which is obliterated at an early stage. Glabellar furrows 

 are introduced in young stages, and in later stages of development 

 there seems to be no reduction of furrows by their obliteration suc- 

 cessively from the front backward, such as is seen in some of the later 

 trilobites. The glabella occupies the whole length of the cranidium 



